This application is based on Japanese Patent Application No. 11-196568 (1999) filed Jul. 9, 1999, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a printing apparatus and a printing method, more specifically to a printing apparatus and a printing method in which a printing head provided with a printing element arrangement of a plurality of printing elements is scanned in a direction different from the above arrangement direction relative to the printing medium, and printing medium (such as paper) transporting of less than the range of the arrangement is performed between respective scans.
2. Description of the Related Art
A so-called serial printer is popularly used, which has a printing element arrangement of a plurality of printing elements arranged in a printing head, and during printing, the printing head is scanned relative to a printing medium to perform printing.
Such printing apparatus can be divided by the printing method into an ink-jet type, a wire dot type, a thermal transfer type, an electro-photographic type and the like. Among these, the ink-jet type printing apparatus ejects ink drops from an ejection opening to the printing medium to perform printing. As the ink ejection method, a bubble jet type, a piezoelectric type or the like is used. In a bubble jet type, an electro-thermal converting element as a thermal energy generation element is provided in an ink passage in the vicinity of the ejection opening, the electro-thermal converting element is driven to cause film boiling of ink in the ink passage, and the ink drop is ejected by a pressure of bubble generated at that time. In the piezoelectric type a piezoelectric element is used as an ejection energy generation element to eject ink utilizing a pressure generated when the piezoelectric element is deformed.
When performing printing by using a printing head having a plurality of printing elements as the above-described ejection openings, quality of the printed image greatly depends on the performance of the single printing head.
For example, in the case of an ink jet type printing head, due to slight differences generated in the printing head manufacturing process, such as deviation of shape of each ejection opening or deviation of formation position of the electro-thermal converting element, ejection amount and ejection direction and the like may be slightly varied between respective printing elements. As a result, differences may be generated such as generation of minute deviation of respective deposition positions of ink drops ejected from respective ejection openings, or variation in formed dot size. This leads to generation of density irregularity or unevenness such as stripes in the image, resulting in deterioration of image quality.
It is known that such deviation in printing characteristics by printing elements is not limited to the ink jet type printing head, but may occur in printing heads of other types.
In order to prevent density variation causing deterioration of image quality, it is proposed to reduce deviation of respective printing characteristics to respective printing elements, for example, using a method in which the same part of the printing medium is scanned for a plurality of times to complete an image, that is, a so-called multi-scanning or multi-pass printing method.
That is, in ordinary printing method, after a printing by scanning the printing head one time, paper is transported by the width of the printing head, and printing of the next region is made. Therefore, the same region of the printing medium is never scanned for a plurality of times by the printing head. Consequently, for example, when there is a deviation in printing characteristic per every printing element, the deviation appears, as is, as density irregularity in the image.
In the multi-scanning printing method, on the other hand, the printing head is divided into a plurality of blocks each of which has several printing elements, and paper transporting is performed according to a width of the divided one block, and the same region of the printing medium is scanned by the printing head for a plurality of times, and at every scanning, the printing elements of different blocks are driven. For example, as shown in FIGS. 12A and 12B, when sixteen printing elements of the printing head are divided into four blocks, the same region is scanned four times to complete the image. Therefore, even when there is deviation in printing characteristics among respective printing elements, printing elements for printing each region are dispersed, influence of difference in printing characteristics on respective regions are reduced, and density unevenness becomes inconspicuous.
As described above, by the multi-scanning printing method, density unevenness due to deviation of printing characteristic among printing elements in the printing head can be reduced in units of scanning regions of the printing head.
However, there may be a case where the printed image differs in density and tint between scanning regions of the printing head. As a result, stripes (hereinafter referred also to as xe2x80x9cbanding artifactsxe2x80x9d) are generated with a period of the width of the scanning region over the entire image, resulting in deterioration of image quality.
One of the major causes of difference in density between scanning regions is due to the order of printing on scanning, when printing is completed by performing scanning of a plurality of times in each region.
For example, in divided printing such as the multi-scanning printing method, in a method where so-called bi-directional printing is effected, density slightly differs between the region printed first by forward scanning and the region printed first by backward scanning, and banding artifacts may occur. In the example shown in FIG. 12B, density differs in respective scanning regions a, b and c, which appear as banding artifacts.
In particular, when performing color printing by depositing ink drops of respective colors overlappingly, the overlapping order of respective color inks differs because of the above bi-directional printing, and banding artifacts are liable to occur.
That is, when ink of a different color is ejected overlappingly on the formerly deposited ink, in the overlapping part, the later deposited ink tends to sink in a depth direction of the paper surface. This phenomenon generates because bonding of the printing medium and dyestuff is limited. That is, because bonding of the dyestuff of the formerly deposited ink and the printing medium has preference, the dyestuff of the latterly deposited ink is difficult to bond on the surf ace of the printing medium, and sinks in the depth direction of the paper surface. As a result, even when printing of a predetermined color is performed by overlapping inks of the same color set, because the overlapping order differs between scanning regions, differences in density or tints are generated, and the above banding artifacts may occur.
A method is proposed as a measure against the above banding artifacts, for example, in the multi-scanning printing method, by increasing the number of division, the formed bind pitch (period of density unevenness) is shortened to make the stripes imperceptible to human eyes, thereby making banding artifacts inconspicuous.
However, in this method, the paper transporting width at a time is reduced, the number of scanning times is increased, and the time required for the entire printing becomes long, which impedes high-speed printing.
An object of the present invention is to provide a printing apparatus and printing method for forming an image of inconspicuous banding artifacts by performing printing by a reduced number of scanning times and by a bind pitch of a period which is imperceptible to human eyes.
In the first aspect of the present invention, there is provided a printing apparatus for performing printing using a printing head having a plurality of printing elements arranged thereon, comprising:
scan means for scanning the printing head relative to a printing medium in a direction different from the arrangement direction of the printing elements;
transport means for transporting the printing medium relative to the printing head by an amount corresponding to a printing element arrangement length in a block of printing elements, the length being obtained by dividing the plurality of printing elements into a plurality of blocks; and
control means for controlling the scan means to perform scanning of a plurality of times and interposing the transporting by the transport means between each scanning, and for completing printing on a region of the printing medium corresponding to the respective plurality of blocks,
wherein the control means controls a printing operation on a partial region as a part of the region in the transporting direction in each scanning of the plurality of times, and completes the printing of the region corresponding to the printing element arrangement length in the block.
In the second aspect of the present invention, there is provided a printing method for performing printing using a printing head having a plurality of printing elements arranged thereon, comprising:
a scan step of scanning the printing head relative to a printing medium in a direction different from the arrangement direction of the printing elements;
a transport step of transporting the printing medium relative to the printing head by an amount corresponding to a printing element arrangement length in a block of printing elements, the length being obtained by dividing the plurality of printing elements into a plurality of blocks; and
a control step of controlling the scan step to perform scanning of a plurality of times and interposing the transporting in the transport step between each scanning, and of completing printing on a region of the printing medium corresponding to the respective plurality of blocks,
wherein the control step controls a printing operation on a partial region as a part of the region in the transporting direction in each scanning of the plurality of times, and completes the printing of the region corresponding to the printing element arrangement length in the block.
With the above construction, since printing elements for performing printing of one scan in one block are only part of the region, formed bind pitch is reduced to less than paper transport width, and only the bind pitch can be reduced without increasing the number of scans. Therefore, when the bind pitch is set to less than sight characteristic, banding artifacts are imperceptible.
The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.